An exposure apparatus which transfers the pattern of a master such as a reticle onto a photosensitive material applied to a substrate such as a wafer or glass plate is used to manufacture a device such as a semiconductor device or liquid crystal display device by photolithography.
In general, a photosensitive material applied to a wafer has a predetermined proper exposure amount. In a conventional exposure apparatus, a beam splitter is arranged in an illumination optical system for illumination light. The light quantity of part of the illumination light split by the beam splitter is monitored by a photoelectric sensor (integrated exposure amount sensor), thereby indirectly monitoring the exposure amount of the wafer. When the exposure amount of the wafer reaches a proper exposure amount, exposure to the current shot region of the wafer is stopped to control the exposure amount.
In such an exposure apparatus, the relationship between the illuminance on the wafer and an output from the integrated exposure amount sensor in the illumination optical system must be measured in advance. For this measurement, a photoelectric sensor for measuring the illuminance on the wafer is generally set on a stage which holds the wafer. The photoelectric sensor on the stage is often used to measure the illuminance uniformity of exposure light incident on the wafer via a projection optical system, and is generally called an illuminance uniformity sensor.
The illuminance uniformity sensor is generally a single photodiode (light-receiving element or photoelectric converter), or a photodiode array or CCD (Charge Coupled Device) comprised of a plurality of photodiodes. A line or area type photoelectric sensor such as the photodiode array or CCD stores charges proportional to the incident light quantity output from the photodiode in a charge storage (capacitor) within the photoelectric sensor. Stored charges are read from the charge storage in accordance with a read command, converted from a current into a voltage, and used for various processes.
To calibrate an output from the illuminance uniformity sensor, an illuminance meter calibrated in advance is set below the projection optical system instead of a wafer, and the illuminance is measured by the calibration illuminance meter. The illuminance uniformity sensor to be calibrated is then moved below the projection optical system, the illuminance is similarly measured by the illuminance uniformity sensor, and an output is so adjusted as to be equal to an output from the calibration illuminance meter.
As a method of checking whether exposure amount control is correctly executed, a predetermined exposure amount is set, and the illuminance uniformity sensor is moved below the projection optical system instead of a wafer. In this state, while exposure amount control is executed on the basis of an output from the integrated exposure amount sensor, an exposure amount actually incident on the illuminance uniformity sensor is measured.
It is a recent trend to use an excimer laser source as an exposure light source in an exposure apparatus which sequentially exposes a plurality of shot regions on a wafer by a step & repeat method using a so called stepper. The excimer laser typically has an energy dispersion of about 10% for 3s between output pulses. To achieve a desired exposure amount precision of, e.g., 1% using a light source having such an energy dispersion, a wafer must be irradiated with at least 100 pulses to perform integrated exposure. For a small target exposure amount, the illuminance is decreased by a beam attenuation means set in the illumination optical system so as to make an actual exposure amount fall within the tolerance of the target exposure amount by integrated exposure of, e.g., 100 pulses.
However, the conventional arrangement requires a wide dynamic range for the integrated exposure amount sensor in the illumination optical system or the illuminance uniformity sensor on the stage. This is because these photoelectric sensors must measure light quantities ranging from a large light quantity which is not attenuated and is used for a large exposure amount to a small light quantity which is attenuated by the beam attenuation means and used for a small exposure amount. The front surface of the photoelectric sensor is covered with the beam attenuation means which adjusts the light quantity such that an optimal light quantity is incident on the photoelectric sensor. In general, the beam attenuation means is so set as not to saturate an output from the photoelectric sensor even if a maximum light quantity is incident on the photoelectric sensor. When the exposure amount is set small and a light quantity incident on the photoelectric sensor decreases, an output from the photoelectric sensor greatly decreases. As a result, the measurement precision decreases under the influence of noise by the dark current of the photoelectric sensor itself, thermal noise, and the linearity between the incident light quantity and output of the photoelectric sensor.
When the illuminance uniformity sensor adopts a line or area type photoelectric sensor such as a photodiode array or CCD comprised of a plurality of light-receiving elements (photoelectric converters), a long read time is taken to read output signals from all the light-receiving elements. The emission frequency of an excimer laser has recently been increased, and lasers having an emission frequency of 4 kHz or more become available. Such a high-frequency laser has a short time interval between emission pulses, and it becomes difficult to read output signals from all the light-receiving elements within this time interval.